Optical amplifiers are important in networks for distributing optical signals. Optical fiber amplifiers, made from glass that is doped with rare earth elements such as erbium, are a well-known example. For example, U.S. Pat. No. 4,826,288, issued to R. J. Mansfield, et al. on May 2, 1989, describes one method for fabricating optical fibers having cores with relatively high rare earth content. However, for applications such as premises distribution of optical signals, where components need to be relatively small and device integration is desirable, it is advantageous to provide optical amplifiers in the form of planar waveguides deposited on silicon substrates. However, as currently envisioned, such devices are much shorter than optical fiber amplifiers, and the required level of doping is correspondingly much greater.
Attempts to deposit layers of glass at the high doping level appropriate for this purpose have been generally unsatisfactory. For example, a doped soot layer can be formed by chemical vapor deposition, and subsequently sintered to form a glass layer. A method for forming glass waveguides is described, for example, in U.S. Pat. No. 4,425,146, issued to T. Izawa, et al., on Jan. 10, 1984. A process for making sintered glasses containing rare earth dopants is described, for example, in U.S. Pat. No. 4,826,288, issued to R. J. Mansfield, et al., on May 2, 1989. However, the sintering temperature required, which may be as much as about 1200.degree. C., may promote phase separation and may damage underlying structures on the substrate. As a consequence, it would be advantageous to find a core glass composition that can be doped with erbium and incorporated in a waveguide amplifier without exposing the waveguide structure to potentially damaging, high temperatures.